Apparatus for controlling well pressure

ABSTRACT

Apparatus is disclosed for controlling the bottom hole pressure of a well into which a drill string extends by automatically regulating a choke at the outlet of the well.

United @tates Patent 1191 Jones Aug. 6, 1974 APPARATUS FOR CONTROLLHNG WE LL 3,443,643 5/1969 Jones 175/25 PRESSURE 3,470,971 /1969 Dower 175/25 3,470,972 10/1969 Dower 175/25 [75] Inventor: Marvin R. Jones, HOuStOn, TeX. 3,477,526 11/1969 Jones et a]. 175/25 3,517,553 6/1970 Williams et a1. 175/25 [73] Assgnee' Camem 3,588,804 6/1971 Fort 1. 340/18 LD Houston, Tex.

1 1 Filedi 1972 Primary Examiner-James A. Leppink [21 App]. No.: 315,804 Attorney, Agent, or Firm-W. F, Hyer; Marvin B.

Eickenroht [52] US. Cl. 175/25, 166/75, 340/18 LD 511 1111. C1 E2lb 7/00 [571 ABSTRACT Field of Search 175/25, 38, 1 Apparatus is disclosed for controlling the bottom hole 166/75; 340/18 18R; 137/5157; 73/151 pressure of a well into which a drill string extends by automatically regulating a choke at the outlet of the [56] References Cited UNITED STATES PATENTS 3,429,385 2/1969 Jones et a1. /25 10 Claims, 2 Drawing Figures APPARATUS FOR CONTROLLING WELL PRESSURE This invention relates generally to the control of the pressure of drilling mud within a wellbore having a drill string extending into the bore. More particularly, it relates to improvements in apparatus for automatically controlling the well pressure by regulating a choke to impose a desired back pressure on the outlet of the well.

It has been customary to provide a choke in a manifold connecting with the annulus of the well beneath a blow-out preventer so that, upon closing the preventer about the drill string, the choke can establish and maintain a back pressure on the drilling mud diverted through the manifold. This back pressure, together with the hydrostatic pressure of the drilling mud in the well, is intended to contain the pressured fluids within formations penetrated by the wellbore i.e., prevent them from flowing into the wellbore. The choke is preferably adjustable so that, in the case of a kick," it may be regulated in an attempt to maintain a predetermined pressure differential between the bottom hole pressure of the mud and the pressure of the formation fluid as heavier mud is circulated down the drill string and up the annulus to kill the well. During this time, it is desirable not only to contain such fluid, but also to avoid excessive back pressure which might cause the drill string to stick, or damage a formation, the well casing, or the wellhead equipment.

In one use of prior apparatus for this purpose, when a kick is encountered, the preventer and choke manifold are closed, the mud pumps are stopped, and shutin pressure is observed at the manifold upstream of the choke. The pump is then started slowly as the choke is gradually opened to maintain a back pressure at a level slightly above the observed shut-in pressure. When the desired circulating rate is reached, it is held constant and the choke is continuously adjusted to maintain the pressure in a standpipe connected to the upper end of the drill string at the level it has reached at such circulating rate. The constant pressure in the standpipe is maintained until the kick is circulated out of the annulus.

The mud density increase necessary to contain the formation fluid is calculated, and the denser mud is pumped into the drill string with the choke being adjusted to maintain the annulus back pressure constant. When the heavier mud reaches bottom, the user begins to adjust the choke to again maintain the standpipe pressure constant as such mud circulates up through the annulus. Thus, in effect, the user maintains a constant bottom hole pressure by controlling the pressure in that portion of the well where the average density of the fluid in it is known more closely. However, the presence of evolving and expanding gas in the mud in the annulus, and the large density-induced and rateinduced changes in circulating pressure loss in the drill string, complicate the use of such apparatus.

U.S. Pat. Nos. 3,429,385 and 3,477,526, which are assigned to the assignee of the present invention, disclose apparatus in which the circulating pressure loss of the drilling mud is taken into account in inferentially determining bottom hole pressure, so that it is not necessary to maintain a constant circulating rate during control of the well. For this purpose, a device is installed in the standpipe or inlet to the drill string for continuously sensing the circulating pressure loss, and the choke is regulated in such a manner as to maintain a signal representative of the pressure in the standpipe equal to a signal representative of the standpipe pressure when the well is shut-in (static pressure) plus the circulating pressure loss and the desired differential.

However, since this prior apparatus senses the density of the drilling mud at its input and assumes that the sensed density exists throughout the drill string, it introduces an error when the new drilling mud of increased density first reaches the sensor, and this error must be corrected in most cases by manual adjustment of the apparatus. Also, during the time that mud of a new, higher density is displacing the original fluid within the drill string, the hydrostatic pressure increases, so that the operator must continually adjust the apparatus.

It is the primary object of this invention to provide relatively simple and inexpensive apparatus of this type which avoids the necessity of inferentially determining the bottom hole pressure.

Still further, in such prior apparatus, there is a tendency for the choke to hunt or overcontrol due to the time lag between an adjustment at the choke and a response at the inlet. In order to reduce this tendency, it has been proposed, as shown in the apparatus of U.S. Pat. No. 3,477,526, to provide means for damping the signal to be transmitted to the choke operator, and it is another object of this invention to provide apparatus of this type in which there is less time lag between the choke and transducer, and thus less need for correction.

These and other objects are accomplished, in accordance with the illustrated embodiment of the invention, by apparatus which includes a means for sensing and thus directly determining ambient fluid pressure near the lower end of the drill string, a means for comparing the sensed pressure with a desired pressure, and a means responsive to a difference between the sensed and desired pressures for operating the choke in a manner to reduce such difference. In this way, it is possible to maintain a predetermined pressure differential between the bottom hole pressure and formation pressure without having to inferentially determine such bottom hole pressure.

More particularly, this means for directly determining ambient pressure near the lower end of the drill string includes means for producing a first signal representative of such sensed pressure and transmitting such signal back to the surface. This may be accomplished in a number of different ways. Thus, SPE Paper 1843 and World Oil article, December 1967, New Concept in Drilling Rigs Designed to Lower Well Costs, describe non-rotating drilling pipe having both power and signal conductors, which enable a pressure sensing device to be installed on the inner or outer side of the drill string near its lower end. In the more conventional well drilling situations, where a rotating drill string is employed, a number of prior patents, i.e., U.S. Pat. Nos. 3,205,477; 3,309,656; and 3,588,804, suggest acoustic or sonic apparatus for telemetering information signals, including signals providing pressure information, from the bottom of the hole. In U.S. Pat. No. 2,370,818, a wire line communication system is illustrated which can be adapted to be connected to a pressure sensing device or transducer at the bottom of the hole being drilled. In the latter case, it may be possible to employ additional apparatus for inserting a cable into a well under pressure, such as shown in Composite Catalog of Oilfield Equipment and Services, 23th Revision, page 751, to aid in lowering of the transducer through the drill string to a position in the drill string near its lower end.

The apparatus of this invention also includes means for producing a second signal representative of a desired fluid pressure near the lower end of the drill string, which would include a pressure differential, positive or negative, by which the bottom hole pressure of the drilling fluid is to exceed the formation fluid pressure. Still further, means are provided for comparing these first and second signals and producing a control signal which represents the difference between them, and for operating the choke in response to the control signal so as to increase or decrease the outlet fluid pressure (i.e., upstream of the choke) in such a manner that it approaches a value at which the desired pressure differential is maintained.

In the illustrated embodiment of the invention, the choke operating means includes a reciprocable, pressure responsive actuator connected to the choke, means for sensing the actual outlet fluid pressure, means for producing a third signal representative of the sensed outlet pressure, and means for opposing the third signal to the control signal, whereby the third signal acts as a bias to cause the choke to operate in the manner described by increasing or decreasing the outlet pressure in response to an imbalance between it and the control signal. Also, a means is provided for producing a fourth signal representative of the desired fluid pressure at the outlet, and the choke operating means includes means for alternately comparing the third and fourth signals, so as to operate the choke in such a manner as to maintain the actual outlet pressure at the desired level.

Inasmuch as the effect of adjustment of the choke is detected by the pressure sensing device near the lower end of the drill string, the time lag and thus the tendency toward hunting or overcontrol is less than that encountered in the prior apparatus. Nevertheless, in order to reduce this tendency still further, a means is preferably provided for damping the control signal.

In the drawings, wherein like reference characters are used throughout to designate like parts:

FIG. I is a diagrammatic illustration of apparatus constructed in accordance with a typical embodiment of the present invention, in which a Kelly has been replaced by equipment for controlling a kick, and a pressure sensing device has been lowered into the lower portion of a rotary drill string, the scale of the lower part of the figure being smaller than that of the upper; and

FIG. 2 is a view of the lower portion of a non-rotating drill string having the pressure sensing device on its outer side.

With reference now to the details of FIG. I of the drawings, the well on which the control apparatus is installed includes a casing 110 lining a portion of a wellbore 11 and a casing head 12 connected to its upper end at the surface level 13. A blowout preventer 14 connected above the casing head 12 has a bore therethrough forming a continuation of the wellbore and easing head and rams l mounted for reciprocation between positions opening and closing the bore.

A drill bit 16 is connected to the lower end of a drill string 17 extending through the wellhead and into the wellbore. The bit normally has orifices in the bottom thereof through which drilling fluid is jetted into the wellbore, and is rotated with the drill string in any well known manner. A velocity valve 17A, which may be similar to that shown in US. Pat. No. 3,027,914, is installed in its lower end.

A Kelly (not shown), which occupies a position at the upper end of the string during normal drilling operations, has been replaced by a circulating head 18 of conventional construction connected to the upper end of the string, and a cable inserting device I9 similar to that above-mentioned installed above the head 18. Drilling mud is circulated through a side inlet 18A in the head 18 for passage downwardly through the drill string, out the orifices in the bit, and upwardly through the annulus between the string and wellbore.

A side outlet 20 connects with the bore of the casing head 12 beneath the blowout preventer, so that with the blowout preventer rams closed about the drill string, as shown in the drawings, drilling fluid within the annulus is diverted into the outlet. The outlet forms part of conventional manifolding for conducting the drilling fluid to one of a plurality of valves and chokes, including an adjustable choke 21 forming part of the apparatus of this invention. Thus, the well may be completely shut in, or drilling fluid may be caused to flow through choke 21.

As shown in the drawing, the bit 16 has penetrated a formation F containing fluid under pressure. Normally, this formation fluid is contained by means of the drilling fluid circulated through the wellbore. However, it may be found that the drilling fluid will not contain the pressure formation fluid, in which case the well may kick, as indicated by the ability of the well to flow with the pumps for circulating the drilling fluid shut down. When this occurs, the operator may shut in the well, as above described, and permit the pressure to build up in the annulus and thus in the outlet 20 leading to the choke 2!. After an initial build-up, the pressure in the lower end of the wellbore, and known as bottom hole pressure, will have stabilized at a value required for containing the formation fluid. It will be understood in this respect that bottom hole pressure means the pressure opposite the lower end of the bit 16 of the drill string, which need not necessarily be at the lower end of the wellbore.

In like vein, formation fluid pressure" is formation F pressure corrected, if necessary, for the distance from the bit to such formation.

The choke 21 is ofa construction quite similar to that shown in the above-mentioned prior patents. Thus, it includes a body 22 having a right angle flowway therethrough and a flow restricting member 23 reciprocable therein between maximum and minimum flow restricting positions with respect to opening 23a intermediate the ends of the flowway. The member 23 is caused to reciprocate by means of an actuator 24 mounted on the body 22 and including a piston 25 reciprocable within a cylinder 26 and having a rod 27 extending from the operator and connecting with the restricting member 23. The piston is caused to reciprocate in response to fluid pressure differential across it, which is in turn responsive to signals to be described.

As illustrated in the drawings, a transducer 28 of any suitable construction is suspended within the lower end of the drill string 17 by means of an electrical cable 29 so as to produce an electrical signal representative of the pressure of fluid at that point and transmit it to a circuitry 29A at the surface. The circuitry includes a source of electrical voltage and a signal amplifier for amplifying the electrical signal, which is then conducted by cable 29 from circuitry 29A to an electric-topneumatic transmitting instrument 298, to be described. Alternatively, and as shown in FIG. 2, the drill string may be of the non-rotating type above mentioned having a transducer 28A near its lower end for sensing pressure on the outside of the string and thus bottom hole pressure and transmitting such signal through suitable conductors in the string to surface level.

In any event, inasmuch as the remaining portion of the control apparatus to be described is pneumatic, the electrical signal is converted to the above-mentioned pneumatic signal by electric-to-pneumatic transmitting instrument. A suitable instrument for this purpose is manufactured by Moore Products Co., of Spring House, Pa., and shown and described in FIG. 4, on page 3 of its Bulletin 7702. The thusly produced pneumatic signal, which may be termed a first signal, is transmitted through line 29C to relay 35, which will be described below. A gauge 30 is connected in line 29C at a convenient location for observing the value of the pressure of fluid within the drill string near its lower end.

An adjustable null balance regulator 32, such as that manufactured by the said Moore Products Co. and

shown on page 14 of its Bulletin 4003, is adapted to produce a pneumatic signal which represents the pressure desired within the string near its lower end, including a desired pressure differential, positive or negative, by which the bottom hole pressure is to exceed the formation fluid. This signal, which may be termed a second signal, is transmitted through line 33 leading from the regulator to relay 35, and a gauge 34 connected in the line provides a visual indication of such desired pressure. Air is supplied through line 31, which has a branch 31A leading to regulator 32 and a branch 31B leading to the instrument 29B.

The relay 35, which compares the first and second signals transmitted to it through lines 29C and 33 and produces a control signal representative of the difference between them, may comprise a pneumatic summing relay of the type manufactured by Moore Products Co., and shown on page 22 of its Bulletin 68A-b 01. As shown in the drawing, the relay 35 is adapted to receive air from a branch 31C of supply line 31, or exhaust it to atmosphere through a port 50, and supply a greater or lesser signal to outlet conduit 53, depending on the relative values of the first and second signals. Thus, if the first signal is greater than the second signal, the relay produces a lesser outlet signal, and if the first signal is less than the second signal, the relay produces a greater outlet signal.

A needle valve 52 is disposed in the conduit 53 to provide a damped control signal for transmission through a line 36 to a hydraulic relay 37 to be described below. The damped control signal is fed back to relay 35 through a conduit 51, and internal porting (not shown) in the relay transmits the undamped outlet signal to relay 35 to oppose the signal fed back, whereby the rate of change of the control signal with respect to time becomes a function of the difference between the first and second signals.

A transducer 38 is connected to the well outlet 20 for sensing the outlet fluid pressure upstream from choke 21 and producing a pneumatic signal representative thereof, which may be termed a third signal. Air is transmitted to the transducer through a branch 31D of supply line 31, and the third signal is transmitted through a line 39 to the hydraulic relay 37 to oppose the control signal, in a manner to be described below. The value of outlet pressure represented by the signal is indicated on a gauge 39A connecting with the line 39.

An adjustable null balance regulator 4i, similar to the regulator 32, is adapted to produce a pneumatic signal which represents the pressure which may be desired within the outlet. The resulting signal, which may be termed a fourth signal, is transmitted through a line 42 from the regulator to a selector valve 55 disposed within the line 36 leading to relay 37. A gauge 42A connected in the line 42 provides a visual indication of the desired outlet pressure. Air is transmitted to the regulator 41 through a branch 31E from supply line 31.

As shown in the drawing, valve 55 is in a position for transmitting the control signal from relay 35 to the relay 37. However, as will be understood from the drawing, such valve may be turned to an alternate position for transmitting the fourth signal from regulator 41 to the relay 37. The purpose of alternatively transmitting either the control or fourth signal to the relay 37 will be understood from the description to follow.

The relay 37 includes a member 37A shiftable within a housing 37B between alternate positions for admitting hydraulic fluid from a suitable source to one side of the actuator 24 and exhausting it from the other, and a neutral position, as shown in the drawing, for interrupting flow between the source and operator. For this purpose, hydraulic fluid is supplied to the relay by a conduit 44 leading from the source and returned therefrom back to the source through a conduit 45. The relay is connected with the right-hand side of the actuator piston 25 by means of a conduit 46 and to the lefthand side thereof by means of a conduit 47. Thus, upon shifting of the shiftable member to the right, hydraulic fluid will be supplied from the line 44 through the line 46 to the right-hand side of the piston 25 and exhausted from the left side thereof through the line 47 to the line 45, for shifting the restricting member 23 of the choke to the left and thus toward minimum flow restricting position. On the other hand, upon shifting the restricting member to the left, hydraulic fluid will be supplied from the line 44 through the line 47 to the left-hand side of the piston 25, and exhausted from the right side thereof through the line 46 to the line 45, for shifting the restricting member 23 to the right and thus toward its maximum flow restricting'position.

The signal representing actual outlet well pressure is transmitted through the line 39 to the left end of shiftable member 37A to urge it to the right, and either the control signal or the fourth signal is transmitted through line 36 to the right end of the shiftable member to urge it to the left. Relay 37 includes centering springs 48 on opposite ends of the shiftable member for normally urging such member to its neutral position shown in the drawing.

With the valve 55 positioned as shown, the relay 37 will shift with a change in the control signal in line 36, so as to adjust the choke in such a manner as to cause the first signal to be equal to the second signal. Thus, in the event the control signal decreases due to an increase in the first signal, shiftable member 37A moves to the right to cause reciprocable member 23 to move away from flow restricting position and thereby decrease the first signal; and, in the event the control signal increases due to a decrease in the first signal, shiftable member 37A moves to the left to cause the reciprocable member to move toward flow restricting posi tion and thereby increase the first signal. In either event, the choke operates to maintain the control signal constant and thus the pressure within the lower end of the drill string at its desired value. On the other hand, with the valve 55 turned to its alternate position, the relay will shift depending on an imbalance between the third and fourth signals, so as to maintain the outlet pressure at its desired value.

The outlet pressure signal has an additional function in that it will cause the relay to respond to abnormal pressure fluctuations in the annulus, even though such changes may not be immediately reflected by a change in the signal representing pressure near the lower end of the drill string. Thus, for example, in the event of an abnormal increase in annulus pressure, the shiftable member of the relay will be caused to move to the right and thus to cause hydraulic fluid to urge the flow restricting member 23 toward minimum flow restricting position, whereby outlet pressure is quickly reduced.

An accumulator 56 is preferably connected in line 36 intermediate the valve 55 and the relay 37. As described in the previously mentioned US. Pat. No. 3,477,526, this too is useful in preventing hunting and overcontrol.

In one manner of using the apparatus, upon indication of a kick, the operator shuts in the well, as previously described, by closing the choke manifold, shutting the pumps down, raising the drill string so as to bring the Kelly above the rotary table, and closing the blowout preventer around the drill string. He then observes the pressures indicated by gauge 30 and/or by gauge 39A to confirm kick by pressure buildup on either, and, if this is the case, performs the following steps:

1. Replaces the Kelly with circulating head 18 and cable inserting device 19,

2. Runs pressure transducer 28 on cable 29 to as near to bottom in the drill string as possible,

3. Sets regulator 41 so that pressure gauge 42A reads the same as gauge 39A,

4. Sets selector valve 55 to transmit the signal produced by pressure regulator 41 to relay 37,

5. Places outlet in open communication with choke 21,

6. Starts the pumps slowly and gradually brings them up to the speed desired for the killing operation,

7. Observes the pressure indicated at gauge 30 and sets regulator 32 so that gauge 34 shows the same pressure as gauge 30,

8. Moves selector valve 55 so as to transmit the output signal from relay 35 to relay 37, and

9. Pumps the wellbore free of formation fluid, displacing the original drilling fluid with one of increased density, as desired.

In another manner of using the apparatus, when the transducer is on the outside of the drill string, so as to sense bottom hole pressure directly as illustrated in FIG. 2, the operator would merely have to perform the fifth through the ninth steps.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the apparatus.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed 1. Apparatus for controlling the bottom hole pressure ofa well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for comparing said sensed pressure with a desired pressure, and means responsive to a difference between the sensed and desired pressures for operating the choke in a manner to reduce said difference.

2. Apparatus for maintaining a predetermined pressure differential between the bottom hole pressure and formation fluid pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for comparing said sensed pressure with an ambient pressure which is required to maintain said predetermined pressure differential, and means responsive to a difference between the sensed and required ambient pressures for operating the choke in a manner to reduce said difference.

3. Apparatus for maintaining a predetermined pressure differential between the bottom hole pressure and formation fluid pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for producing a first signal representative of the sensed ambient pressure, means for producing a second signal representative of an ambient pressure which is required to maintain said predetermined pressure differential, means for comparing said first and second signals, and means responsive to a difference between the signals for operating the choke in a manner to reduce said difference.

4. Apparatus for maintaining a predetermined pressure differential between the bottom hole pressure and formation fluid pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for producing a first signal representative of sensed ambient pressure, means for producing a second signal representative of an ambient pressure which is required to maintain said predetermined pressure differential, means for comparing the first and second signals and producing a control signal representative of the difference between them, and means responsive to said control signal for operating the choke in a manner to reduce said difference.

5. Apparatus of the character defined in claim 4, including means for sensing the pressure of fluid at the outlet, and means for producing a third signal representative of the sensed outlet pressure, said choke operating means including means opposing said control signal to said third signal, whereby the choke operates to re duce said difference by increasing or decreasing the outlet pressure in response to an imbalance between said control signal and said third signal.

6. Apparatus of the character defined in claim 5, including means for producing a fourth signal representative of a desired fluid pressure at the outlet, said choke operating means also including means for alternately comparing said fourth signal with said third signal so as to operate the choke in a manner to maintain the third signal equal to the fourth signal.

7. Apparatus of the character defined in claim 6, wherein said means for sensing ambient pressure near the lower end of the drill string is adapted to sense such pressure on the inside of the drill string.

8. Apparatus of the character defined in claim 4, wherein said means for sensing ambient pressure near the lower end of the drill string is adapted to sense such pressure on the outside of the drill string.

9. Apparatus of the character defined in claim 4, wherein said means for sensing ambient pressure near the lower end of the drill string is adapted to sense such pressure on the inside of the drill string.

10. Apparatus of the character defined in claim 4, in-

cluding means for damping said control signal. 

1. Apparatus for controlling the bottom hole pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for comparing said sensed pressure with a desired pressure, and means responsive to a difference between the sensed and desired pressures for operating the choke in a manner to reduce said difference.
 2. Apparatus for maintaining a predetermined pressure differential between the bottom hole pressure and formation fluid pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for comparing said sensed pressure with an ambient pressure which is required to maintain said predetermined pressure differential, and means responsive to a difference between the sensed and required ambient pressures for operating the choke in a manner to reduce said difference.
 3. Apparatus for maintaining a predetermined pressure differential between the bottom hole pressure and formation fluid pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for producing a first signal representative of the sensed ambient pressure, means for producing a second signal representative of an ambient pressure which is required to maintain said predetermined pressure differential, means for comparing said first and second signals, and means respoNsive to a difference between the signals for operating the choke in a manner to reduce said difference.
 4. Apparatus for maintaining a predetermined pressure differential between the bottom hole pressure and formation fluid pressure of a well, wherein said well has a drill string therein and fluid is circulated therethrough to an outlet from the well, comprising a choke for regulating pressure at the outlet, means for sensing ambient pressure near the bottom of the drill string, means for producing a first signal representative of sensed ambient pressure, means for producing a second signal representative of an ambient pressure which is required to maintain said predetermined pressure differential, means for comparing the first and second signals and producing a control signal representative of the difference between them, and means responsive to said control signal for operating the choke in a manner to reduce said difference.
 5. Apparatus of the character defined in claim 4, including means for sensing the pressure of fluid at the outlet, and means for producing a third signal representative of the sensed outlet pressure, said choke operating means including means opposing said control signal to said third signal, whereby the choke operates to reduce said difference by increasing or decreasing the outlet pressure in response to an imbalance between said control signal and said third signal.
 6. Apparatus of the character defined in claim 5, including means for producing a fourth signal representative of a desired fluid pressure at the outlet, said choke operating means also including means for alternately comparing said fourth signal with said third signal so as to operate the choke in a manner to maintain the third signal equal to the fourth signal.
 7. Apparatus of the character defined in claim 6, wherein said means for sensing ambient pressure near the lower end of the drill string is adapted to sense such pressure on the inside of the drill string.
 8. Apparatus of the character defined in claim 4, wherein said means for sensing ambient pressure near the lower end of the drill string is adapted to sense such pressure on the outside of the drill string.
 9. Apparatus of the character defined in claim 4, wherein said means for sensing ambient pressure near the lower end of the drill string is adapted to sense such pressure on the inside of the drill string.
 10. Apparatus of the character defined in claim 4, including means for damping said control signal. 